The difficulty of conveying sarcasm through writing (in any language) derives from the fact that sarcasm involves an interaction between the linguistic and paralinguistic layers of speech, particularly between pragmatics and prosody.

That's the book-larnin' way of saying that in sarcasm, your prosody (intonations, pauses, beats, essentially the rhythm and music of your voice) contradicts the literal meaning of your speech. Prosody is responsible for lots of other things, too, like letting us know that a sentence was either a question or was emitted from a representative of a particular stereotype, or that the word is import, not import, or a bunch of other uses, it's the Swiss army knife of speech phenomena. While it's not totally lost, prosody is imperfectly reproduced in writing.

Anyway, this gives you a clue on how to best convey sarcasm within the conventions of written English. Punctuation--particularly dashes, and of course, well...ellipsis, are closely related to prosody, and we mustn't forget italics. With the addition of judicious spelling, if you're lucky, you can clue in the reader to the way you would have spoken a sentence.

“Oh, I'm so sorry, I didn't know you were French.”

“Oh, I'm sooooo sorry...I didn't know you were French!

Yes, I study this. No, I'm not kidding.